PART
TWO:
ADVANCED OPERATION AND PROGRAMMING
CHAPTERS
SEQUENTIAL DATA FILES
THE
CONCEPT
OF
FILES
A
file
on
a diskette
is
just like a
file
cabinet in your office--an organized place to put
things. Nearly everything you put on a diskette goes in one kind
of
file
or another. So far
all you've used are program files, but there are others. In this chapter you'll learn about
sequential data files.
The primary purpose
of
a data
file
is to store the contents
of
program variables,
so
they won't be lost when the program ends. A sequential data file
is
one in which the
contents
of
the variabies are stored
"in
sequence," one right after another. You may
already be familiar with sequential files from using a DATASSETTE™, because sequen-
tial
files
on diskette are just like the data files used
on
cassettes. Whether on cassette or
diskette, sequential files must be read from beginning
to
end.
When sequential files are created, information (data) is transferred byte-by-byte,
through a buffer, onto the magnetic media. Once in the disk drive, program files,
sequential data files, and user files all work sequentially. Even the directory acts like a
sequential file.
To use sequential files properly, we willleam some more BASIC words in the next
few pages. Then we'll put them together in a simple but useful program.
NOTE:
Besides sequential data files, two other
file
types are recorded sequentially
on a diskette. They are program files, and user files. When you save a program on a
diskette, it is saved in order from beginning
to
end, just like the information in a
sequential data file. The main difference is in the commands you use to access it.
User
files
can be even more similar to sequential data files. User
files
are almost
never used, but like program files, they could be treated
as
though they were
sequential data files and some can be accessed with the same commands.
For the advanced user, the similarity
of
the various
file
types offers the
possibility
of
reading a program file into the computer a byte (character) at a time
and rewriting it to the diskette in a modified form.
OPENING
A
FILE
One
of
the most powerful tools in Commodore BASIC is the OPEN statement. With
it, you may send data almost anywhere, much like a telephone switchboard.
As
you might
expect, a command that can
do
this much is fairly complex. You have already used OPEN
statements regularly in some
of
your diskette commands.
Before you study the format
of
the OPEN statement, let's review some
of
the
possible devices in a Commodore computer system:
39
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